1. Field of the Invention
The present invention relates to a high density recording medium, and more particularly, to a high density recording medium with a super-resolution near-field structure that is manufactured using a high-melting point metal oxide or silicon oxide mask layer.
2. Description of the Related Art
Conventional recording media can be classified into magneto-optical recording media or phase change recording media. In magneto-optical recording media, such as mini disks (MDs), information is read by detecting the rotation of a straight polarized light reflected from a magnetic film according to the magnetic force and the magnetization direction of the magnetic film. The rotation of the reflected light is known as the “Kerr Effect”. In phase change recording media, such as digital versatile discs (DVDs), information is read based on the difference in reflectivity due to the different absorption coefficients of an optical constant between an amorphous recorded domain and a crystalline non-recorded domain of the recording medium.
Recently, many diversified methods of recording information using micro marks (pits), as in a phase change method, and reproducing information from the recording medium regardless of the diffraction limit have been suggested. The most interested one among these methods is a recording method using a super-resolution near-field structure, which is disclosed in Applied Physics Letters, Vol. 73, No. 15, October 1998, and Japanese Journal of Applied Physics, Vol. 39, Part I, No. 2B, 2000, pp. 980-981. A super-resolution near-field structure utilizes local surface plasmon generated in its special mask layer to reproduce information. The super-resolution near-field structure is classified as an antimony (Sb) transmission type which has an antimony mask layer that becomes transparent by laser irradiation when reproducing information from the recording medium or as a silver oxide decomposition type which has a silver oxide(AgOx) mask layer that decomposes into oxygen and silver, which acts as a scattering source inducing local plasmon.
FIG. 1 illustrates the structure of a recording medium using a conventional super-resolution near-field structure.
As shown in FIG. 1, the recording medium includes a second dielectric layer 112-2 made of, for example, ZnS—SiO2, a recording layer 115 made of, for example, GeSbTe, a protective layer 114 made of dielectric materials, for example, ZnS—Si02 or SiN, a mask layer 113 made of, for example, Sb or AgOx, a first dielectric layer 112-1 made of, for example, ZnS—Si02 or SiN, and a transparent polycarbonate layer 111, which are sequentially stacked upon one another.
When the mask layer 113 is made of Sb, SiN is used for the protective layer 114 and the first dielectric layer 112-1. When the mask layer 113 is made of AgOx, ZnS—SiO2 is used for the protective layer 114 and the first dielectric layer 112-1. The protective layer 114 prevents reaction between the recording layer 115 and the mask layer 113 and is a site upon which a near field acts when reproducing information. When reproducing information, Sb of the mask layer 113 becomes transparent, and AgOx of the mask layer 113 decomposes into oxygen and silver, which acts as a scattering source inducing local plasmons.
The recording medium is irradiated with a laser beam of about 10-15 mW emitted from a laser source 118 through a focusing lens (not shown) to heat the recording layer 115 above 600 C so that a laser-irradiated domain of the recording layer 115 becomes amorphous and has a smaller absorption coefficient k of an optical constant (n, k), regardless of the change of refractive index n of the optical constant (n, k). In an irradiated domain of the Sb or AgOx mask layer 113, the crystalline structure of Sb changes or AgOx irreversibly decomposes, thereby acting as a scattering source which generates plasmon with the result that light of a shorter wavelength than the radiated laser beam is generated. The protective layer 114 serves as a super-resolution near-field toward the recording layer 115. As a result, it is possible to reproduce information recorded on the recording medium as micro marks which are smaller in size than a diffraction limit of the laser used. Therefore, it becomes possible to reproduce information recorded in a high density recording medium using such a super-resolution near-field structure regardless of a diffraction limit of the laser used.
However, in recording media having such a super-resolution near-field structure, their mask layer and recording layer have similar transition temperatures, so ensuring thermal stability during information reproduction is considered as being important. Furthermore, such a super-resolution near-field structure results in poor noise characteristics.